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Have you ever looked at your photos and detected an annoying lack of sharpness? You used a pro-quality lens, you anchored the camera on a sturdy tripod, you were shooting at a low ISO and a moderate shutter speed, you might even have used mirror lockup, and you had your lens stopped all the way down. That recipe has been the standard for most photographers looking to re-create the tack-sharp look of an old-school 4x5 photograph. And, yet, as you print your photo, the whole print just lacked the crispness that you expected. Some shooters faced with this have assumed that they have a sub-par lens or that the lens needs back-focus fine-tuning, but the real culprit is likely to be diffraction.
Most landscape photographers have been imbued with the notion that sharpness from near to far in the frame is necessary. This unspoken rule was born from the legacy of Ansel Adams and the ƒ/64 Group. Created during the 1930s, the group of photographers was reacting to the Pictorialist style that had developed from the early days of the medium. Pictorialism used photography as an extension of painting or drawing. Images were manipulated to be soft or "painterly." The ƒ/64 Group rebelled against this interpretation of photography; instead, they advocated and created images that took advantage of the camera's inherent ability to render a scene sharply. The group took their name from the smallest ƒ-stops that were available on their lenses—that is, the ƒ-stops used to create maximum depth of field and, therefore, sharpness in the photograph.
From the ƒ/64 legacy, many modern landscape shooters use the smallest apertures available on their DSLR lenses, usually ƒ/22. However, shooting at ƒ/22 can introduce the sharpness-robbing phenomenon of diffraction. Basically, diffraction occurs when light waves pass through small openings and the waves spread out on the other side of those openings.
Shot at Point Lobos in California, this photo shows considerable diffraction in the fine details.
If you're thinking that the likelihood of encountering diffraction at ƒ/22 is slim because you're thinking about all of those incredibly sharp Ansel Adams prints that were shot at ƒ/45 or even ƒ/64, consider a few other factors. First, the ƒ number is a ratio where ƒ = the focal length of the lens. On Adams' 4x5 view camera, a 90mm lens is wide angle (approximately equal to a 28mm on a full-frame DSLR or an 18mm on an APS-C DSLR). Plugging in the ratio, a 90mm at ƒ/64 gives an aperture diameter of 1.4mm while a 28mm at ƒ/22 gives an aperture diameter of 1.27mm. That's approximately a 10% difference, which, at such small apertures, can make a considerable difference where diffraction is concerned. Also, most photographers shooting large format at wide angles would tend to keep their minimum ƒ-stop to ƒ/34 or ƒ/45 to limit the influence of diffraction. With the APS-C DSLR and an 18mm lens at ƒ/22, the aperture diameter is miniscule at approximately 0.8mm!
The other aspect to think about is enlargement. A 4x5 film image printed to an 8x10 print is a 4X enlargement. If there's some diffraction present in such a negative, you're enlarging it 4X. A full-frame DSLR photograph printed to 8x10 inches is an enlargement of about 60X. So, if you have even slight diffraction in your DSLR image, you're magnifying it 60X for an 8x10 print.
To limit the effect of diffraction in your photos, your best bet is to test your lenses on the kind of subject matter you shoot most often and print your images at sizes you prefer. A good rule of thumb is to make your minimum ƒ-stop with wide-angle lenses ƒ/16. That should give you plenty of depth of field while reducing diffraction. With longer lenses, you can stop down a bit more.